WO2004082037A2

WO2004082037A2 - A motor combining a resonant vibration mode, a stepping mode and a direct positioning mode

Info

Publication number: WO2004082037A2
Application number: PCT/BE2004/000034
Authority: WO
Inventors: Kristof Decoster; Michael De Volder; Steven Devos; Dominiek Reynaerts; Filips Schillebeeckx; Hendrik Van Brussel; Wim Van De Vijver
Original assignee: K.U. Leuven Research And Development
Priority date: 2003-03-10
Filing date: 2004-03-08
Publication date: 2004-09-23
Also published as: WO2004082037A3

Abstract

Present invention describes a new motor the allows very accurate and fast positioning. The motor comprises a fixed frame (9, 9a), two actuators (6, 7), and a stator (8) that can operate in three operation modes. The first operation mode is called the “resonant vibration mode”, in this operation mode the motor can be classified as a bimodal ultrasonic motor. The second is called the “stepping mode”, in which the motor operates as a walking drive. Finally, the third operation mode is called the “direct positioning mode”, in which the motor can be classified as a flexure nanopositioner. Each of these three operation modes has its own specific characteristics: the resonant vibration mode allows fast, but coarse positioning with a large stroke. The stepping mode allows accurate, but slow positioning with a large stroke and the direct positioning mode allows very accurate and fast positioning with a small stroke. Combining these three operation modes results in a compact, accurate and fast motor.

Description

A MOTOR COMBINING A RESONANT VIBRATION MODE, A STEPPING MODE AND A DIRECT POSITIONING MODE

FIELD OF THE INVENTION

The present invention involves a motor, preferably a piezoelectric motor, capable of operating in a sequence of a resonant vibration mode, a stepping mode and a direct positioning mode or a combination thereof. More particularly it involves a motor that can operate as a bimodal ultrasonic motor, a walking drive and a flexure nanopositioner. The motor can be used as a linear motor, a rotational motor or a planar motor.

BACKGROUND OF THE INVENTION

Recent technologies require positioning systems that are not only precise (for instance a resolution of 1 nm) but that are also fast (more than 100 mm/s). The motor of present invention was found to meet these requirements. The motor uses actuators, preferably piezoelectric, to obtain a high positioning speed of about 1000 mm/s and to achieve an infinite resolution of movement. Piezoelectric actuators also have advantages compared to electromagnetic actuators in specific applications requiring strict environments such as highly vacuumed or magnetic-isolated conditions. Furthermore, motors using piezoelectric actuators are compact and have a high torque at low speeds. The motor of present invention can be used as a linear, rotational or planar drive. When combined with other motors, it can be used as a displacement device with a modular system with displacement units as described in U.S. Pat. 5,898,243. This configuration makes it possible to construct a positioning system that integrates bearings and drives, in order to construct a very stiff and accurate positioning system.

The motor of present invention combines three operation modes: a bimodal ultrasonic mode, a walking-drive mode and a flexure-nanopositioner mode. Although several bimodal ultrasonic motors have been designed over the last years (U.S. Pat. 5,136,200; EP 0 313 072 Al; U.S. Pat. 4,613,782; U.S. Pat. 5,852,336; U.S. Pat. 6,147,436), they cannot be combined with a stepping and the direct positioning mode. U.S. Pat. No. 6,147,436 for instance, describes a piezoactive motor (figure 1) that has similarities in shape to the piezoactive motor of present invention. However, part (4) of this motor is a mobile countermass that is not attached to a fixed frame. Therefore the piezoactuators cannot push against the fixed frame and consequently this motor cannot work as a stepping motor or as a direct positioner. In present invention, however, there is no mobile countermass and instead the actuators are placed between the stator and the fixed frame.

In U.S. Pat. No. 6, 147,436 the mobile countermass (4) is used to make the natural frequencies of the resonant vibration mode coincide. In the motor of present invention, however, the equivalent part (9a) is fixed or is part of the fixed frame (9), and thus cannot be used for this purpose. In the present invention, the shape of the ring, being part 3 in figure 1, was adjusted to let the natural frequencies coincide. Instead of the circular- shaped stator of figure 1, an ellipse-shaped stator (8), as sketched in figure 2, can be used.

Connecting part (9a) to the fixed frame or integrating (9a) in the fixed frame (9) results in the fact that only one single motor can move a mobile unit in stepping mode and direct positioning mode. One single motor as described in U.S. Pat. No. 6,147,436 cannot operate in a stepping mode, since the mobile countermass is not able to transmit the force from the actuators to the fixed frame. For the same reason, one motor of U.S. Pat. No. 6,147,436 cannot support the direct positioning mode.

ILLUSTRATIVE EMBODIMENT OF THE INVENTION

Summary of the invention

The present invention describes a new motor, preferably a piezoelectric motor, which allows very accurate (submicron) and fast positioning (for instance about 1000 mm/s) and of which the motor comprises a stator, an ellipse- shaped stator or an half-ellipse-shaped stator (e.g. as in fig. 8), a single closed structured stator (e.g. as in fig 14), a single half- closed structured stator (e.g. as in fig 15), a double closed structured stator (e.g. as in fig. 16), a double half-closed structured stator (e.g. as in fig. 17) or a triple closed structure (e.g. as in fig. 18) that can operate in three operation modes, i.e. a resonant vibration mode, a stepping mode and a direct positioning mode. Other similar structures are also possible.

The first operation mode is called the "resonant vibration mode", in this operation mode the motor can be classified as a bimodal ultrasonic motor. The second operation mode is called the "stepping mode", in which the motor operates as a walking drive. The third operation mode is called the "direct positioning mode", in which the motor can be classified as a flexure nanopositioner.

Each of these three operation modes has its own specific characteristics: the resonant vibration mode allows fast, but coarse positioning with a large stroke. The resonant vibration mode can for instance allow an infinite stroke with a fast speed until between 0 mm/s to 5000 mm/s, preferably between 0 to 1000 mm/s, but with a relatively coarse positioning resolution, e.g. a resolution of movement of about 1 μm, preferably a movement of about 0.1 to 5mm, more preferably of about 0.5 to 2.5 mm, yet more preferably of about 0.8 to 1.2 mm and most preferably of about 1 mm. The stepping mode allows accurate, but slow positioning with an infinite stroke and the direct positioning mode allows very accurate and fast positioning with a small stroke. The stepping mode can for instance allow a positioning speed of 0 to 20 mm/s, preferably 0 to 2 mm/s with an infinite stroke.

The direct positioning mode can for instance allow positioning with an infinite resolution of movement and a positioning speed between 0 and 1000 mm/s, preferably between 0 and 100 mm/s with a small stroke of 0.5-20 μm, preferably of 1-10 μm.

A motor capable of operating in a sequential combination of these three operation modes results in a compact, accurate and fast motor. Such motor can for instance have a size of 10-100 mm and operate with an infinite resolution of movement and a speed between 0 and 1000 mm/s. Present invention has an advantage compared to motors of the prior art, for instance the motor described in U.S. Pat. No. 6,147,436. The stepping and the direct positioning mode can be achieved with only one stator (preferably an ellipse-shaped stator, or an half- ellipse stator (e.g. as in fig. 8), a single closed structured stator (e.g. as in fig 14), a single half-closed structured stator (e.g. as in fig 15), a doubled closed structured stator (e.g. as in fig. 16), a double half-closed structured stator (e.g. as in fig. 17) or a triple closed structure (e.g. as in fig. 18)) and two actuators, preferably piezoelectric actuators. In U.S. Pat. No. 6,147,436 two ring shaped stators and four piezoelectric actuators are needed for a stepping actuation. Consequently, the motor of present invention can be made more compact and cost-effective. This motor is also extendable to a 2D-version. Furthermore, when combined with other motors, the bearing function can be incorporated, as described in U.S. Pat. 5,898,243. This way a positioning system combining bearing function and drive function can be constructed. Because the height of the contact point can be independently controlled, active control of the bearing is possible so an ultra stiff positioning system can be constructed. Embodiments of such positioning systems are given in figs. 10, 11 and 12. Figure 10 shows a positioning system with 2 degrees of freedom of translation (X,Y) and one of rotation around a vertical axis (C). Figure 11 shows a spindle with one degree of freedom of translation (X) and one of rotation (A). Figure 12 shows a Stewart platform with 3 degrees of freedom of rotation (A,B,C).

Brief description of the drawings

Figure 1 is a view of a design of a motor of the prior art (U.S. Pat. No. 6,147,436).

Number 1 in this figure is the actuator, number 2 = actuator; 3 = stator; 4 = mobile countermass and 5 = flexible leaf springs

Figure 2 is a view of an actuator of present invention. Numbers in this figure: 6 = actuator; 7 = actuator, 8 = stator; 9 and 9a = fixed frame; 10 = contact line and 11 = leaf springs

Figure 3 demonstrates the direct positioning mode wherein Figure 3.i) demonstrates the neutral position (Actuator (6) and (7) are neutral); Figure 3.ii) demonstrates the actuation to the left (actuator (6) is expanded, actuator (7) is retracted), Figure 3.iii) demonstrates actuation to the right (actuator (6) is retracted, actuator (7) is expanded).

Other numbers in this figure: 8 = stator; 9 = fixed frame and 13 = slider.

Figure 4 demonstrates the stepping mode of the motor of present invention, wherein:

Figure 4.i) demonstrates the start position (actuator (6) is expanded, actuator (7) is retracted); Figure 4.ii) demonstrates detachment of the contact point (actuator (6) and

(7) are expanded); Figure 4.iii) demonstrates contact of slider and ring moved over ΔL

(actuator (6) is expanded, actuator (7) is retracted) and Figure 4.iv) demonstrates the end position (=start position but slider moved over ΔL)

Other numbers in this figure: 8 = stator, 9 = fixed frame and 13 = slider.

Figure 5 demonstrates the used resonant vibration modes of the motor of present invention. Numbers in this figure: 6 = actuator; 7 = actuator, 8 = stator and 9 = fixed frame.

Figure 6 demonstrates the cycle of the resonant vibration mode of the motor of present invention. Numbers in this figure: 8 = stator, 9 = fixed frame and 13 = slider.

Figure 7 displays the planar version of the combined motor of present invention, wherein part 27 = the contact surface, parts 28 are actuators, parts 29 are leaf springs, parts 30 are tuning blocks, part 31 is the fixed frame, part 32 is the stator.

Figure 8 demonstrates active height control. Numbers in this figure: 6 = actuator; 7 = actuator, 8 = stator, 9 = fixed frame.

Figure 9 is a top view of a slider with six supporting modules. Numbers in this figure: 12

= motor and 13 = slider.

Figure 10 displays an embodiment of present invention wherein the motor (12) is a combined drive and active bearing system for the mobile unit (13), i.e. an XY-positioning system. Other numbers in this figure: 33 = base surface, 34 = contact element.

Figure 11 displays an embodiment of present invention wherein the motor (12) is a combined drive and active bearing system for the mobile unit (13), i.e. a spindle. Other numbers in this figure: 33 = base surface, 34 = contact element.

Figure 12 displays an embodiment of present invention wherein the motor (12) is a combined drive and active bearing system for the mobile unit (13), i.e. a Stewart platform. Other numbers in this figure: 33 = base surface, 34 = contact element. Figure 13 demonstrates a mobile unit, which is supported by a separate bearing system.

The motor drives the contacting mobile unit. Numbers in this figure: 6 = actuator; 7 = actuator, 8 = stator, 9a = fixed frame and 13 = slider.

Figure 14 demonstrates a stator of present invention as a single closed structure (14)

Figure 15 demonstrates the stator as a single half-closed structure (15)

Figure 16 demonstrates the stator as a double closed structure (16)

Figure 17 demonstrates the stator as a double half-closed structure (17)

Figure 18 demonstrates the stator as a triple closed structure (18)

Figure 19 is a view of an embodiment of present invention wherein the actuators are mounted inside the stator, wherein 19 = actuator; 20 = actuator; 21 - stator and 22 = fixed frame.

Figure 20 is a view of an embodiment of present invention wherein the actuators are mounted outside the stator, wherein 23 = actuator; 24 = actuator; 25 = stator and 26 = fixed frame.

Figure 21 shows the Frequency Response Functions (FRF) defined as the ratio of the excitation signals and the motion of the contact point for the vertical mode and the horizontal modes in X and Y-direction for a planar module.

Figure 22 shows the relation between the driving signals and motion of the contact point in the resonant vibration mode for a planar module.

Figure 23 shows the motion of the contact point for different speeds of the slider, while maintaining the thrust force.

Figure 24 shows the motion of the contact point for different thrust forces of the slider, while maintaining the slider speed.

Brief Description of the invention

Description of the actuator

As sketched in figure 2, the motor comprises a stator, preferably an ellipse-shaped stator or an half-ellipse stator (e.g. as in fig. 8), a single closed structured stator (e.g. as in fig 14), a single half-closed structured stator (e.g. as in fig 15), a doubled closed structured stator (e.g. as in fig. 16) ,a double half-closed structured stator (e.g. as in fig. 17) or a triple closed structured stator (e.g. as in fig. 18) said stator being connected to the fixed frame by springs. The stator can be a monolithic structure manufactured by EDM (Electron Discharge Machining) or SLS (Selective Laser Sintering) or any other means. The piezoelectric actuators are built in between the stator and the central fixed frame, either inside the stator (figure 19) or outside the stator (figure 20). Applying the same signal to the piezoactuators results in a vertical motion of the contact point while opposite signals result in a horizontal motion of the contact point. By pressing a slider, wheel or roller ball onto the stator at the contact point, a linear, respectively a rotational motion can be achieved. The case of a linear motor can be seen in figure 2 and 3. The signals applied to the actuators, preferably piezoelectric actuators determine the operation mode. Three operation modes can be distinguished. In the following section, each of these modes will be discussed in detail. Combination of more motors can be used to build a stiff and accurate positioning system, as described in U.S. Pat. 5,898,243.

The operation modes

Direct positioning mode

In the direct positioning mode, the actuator can be classified as a flexure nanopositioner. This mode allows actuating the slider over a short stroke with a high positioning resolution of movement, which is very interesting for precision applications. The actuator operates as follows: first, a slider is pushed onto the stator, then, by expanding one of the actuators and retracting the other one, the slider, wheel or roller ball is actuated. The generated displacement is proportional to the applied voltage. This actuation is based on friction between the stator and the slider, wheel or roller ball. The operation principle is illustrated in figure 3.

This operation mode can have an infinite resolution of movement, but its stroke is limited to a few μm.

Stepping mode

The stroke of the direct positioning mode is too short for many positioning applications, therefore a second operation mode of the actuator is desired. This second operation mode is called the stepping mode, in which the motor operates as a walking drive. This principle makes an infinite stroke possible. The stepping mode is based on the inchworm principle as sketched in figure 4.

The stepping sequence of figure 4 is possible if bearings or other motors, preferably piezoelectric motors, support the slider, wheel or roller ball, otherwise the slider, wheel or roller ball will not detach from the motor in figure 4.ii). The combination of a direct positioning and a stepping mode allows actuation with a high resolution of movement and an infinite stroke, but is relatively slow, typically less than 10 mm/s. This slow actuation is due to the small stroke of the piezoactuators, which limits the maximum step length to a few μm. The stepping frequency is limited in speed because a high frequency, e.g. a frequency of >100 Hz requires high currents e.g. a current of >1 A and in addition causes vibration problems. However, limitation in speed (< lOmm/s) in the stepping mode are in the motor of present invention, however solved by the resonant vibration mode which does not require high currents for high frequencies.

Resonant vibration mode

Many applications require fast actuation. Therefore the actuator of present invention is also designed to operate in a resonant mode. In the resonant vibration mode, the motor can be classified as a bimodal ultrasonic motor. As in all bimodal ultrasonic motors, the two resonant vibration modes, shown in figure 5, are excited at the same frequency so that the contact point describes an amplified elliptical motion. This cycle is shown in figure 6. The elliptical motion of the contact point allows actuating the slider (see figure 6, bottom). The most challenging part of the research is to design the stator so that the two natural frequencies are in the same frequency region. FE and modal analysis play an important role in the design of the actuator. This operation mode allows driving speeds of several hundreds of mm/s.

Driving signals in the resonant vibration mode

The driving signals to obtain a desired motion of the contact point will be explained for a planar version of the motor. The driving signals for the other versions are analogous. The contact point performs an elliptical motion to drive the slider that is pushed against the contact point. When the desired driving direction is the X-axis, the necessary generated motion of the contact point is in the XZ-plane. The desired driving direction can be any direction in the XY-plane, therefore the necessary generated motion of the contact point is in the plane, determined by this direction and the Z-axis.

The speed of the motor can be adjusted by the horizontal amplitude of the elliptical motion and the thrust force can be adjusted by the vertical amplitude of the elliptical motion. To obtain a large vertical and horizontal stroke of the contact point with the given limited stroke of the piezoactuators, amplification by mechanical resonance is used. Therefore, the drive is designed to have two horizontal and one vertical eigenmode in the same frequency region. Figure 21 shows Frequency Response Functions (FRF) between the excitation signals and the corresponding motion of the contact point. For a symmetrical structure, the eigenfrequencies and damping ratios of the two horizontal modes will coincide.

When the four piezoactuators excite the three eigenmodes in that frequency region, the contact point performs an amplified elliptical motion in space. The vertical eigenmode is excited when two or more opposing piezoactuators are driven in phase, a horizontal eigenmode is excited when two or more opposing piezoactuators corresponding to the horizontal direction are driven in anti phase.

This is formulated in Figure 22 by formula (1) for the X-direction and formula (2) for the

Y-direction.

Where:

Pi — P e ' is the phasor representation of the excitation voltage to piezoactuator number i for i =1,2,3,4 with i =voltage amplitude , θ = phase shift, f = frequency, t = time.

Thus the real excitation voltage to for example piezoactuator 1 can be expressed as

Re(p₁) = P_l cos(2πft + θ_l) .

V_x is the phasor representation of the excitation signal of the vertical mode by the piezoactuators in X-direction

V_y is the phasor representation of the excitation signal of the vertical mode by the piezoactuators in Y-direction

H_x is the phasor representation of the excitation signal of the horizontal mode in X- direction by the piezoactuators in X-direction

H_y is the phasor representation of the excitation signal of the horizontal mode in Y- direction by the piezoactuators in Y-direction

Working at resonance implies an amplitude amplification and a phase shift of the resulting motion versus the excitation voltage (see Figure 21), which is expressed in phasor notation in formula (3) and (5) in Figure 22 for the vertical mode, as formula (4) for the horizontal mode in X-direction and formula (6) for the horizontal mode in Y- direction.

This formulation leads to the following relation between the input voltages to the four piezoactuators and the resulting elliptical motion of the contact point:

or:

* = A, (/)• ∞<2πf.t + θ_{l +} φ_hx ( )) - /^> (/). cos(2 + θ₃ + φ_hx ( )) = £ , (/)•

(/)) - P₄ Λ_hy (f). cos(2πf.t + θ₄ + φ_hy (/)) z = z_x + z_y = P_x Λ_v (f). ∞s(2πf.t + θ_{l +} φ_v ( )) + ₂ A (/)• ∞s(2πf.t + θ₂ + φ_v ( )) + ₃ ( )- ∞s(2^ + ^ + P_v (/)) + £ A (/)-∞s(2 ^".f + 0₄ + p_v(/))

This formula shows that the motion of the contact point can be adjusted by the amplitudes P_λ,P₂,P₃, P₄ and the phases θ_λ,θ₂,θ₃, θ₄ of the four piezoactuators.

Independent control of thrust force and speed

An important aspect of the invention is the independent control of the vertical amplitude and the horizontal amplitude of the ellipse by controlling the amplitudes and phase shifts of the input voltages to the piezoactuators. This follows naturally from the theoretical deduction depicted in scheme 1. The vertical amplitude determines the thrust force of the motor, while the horizontal amplitude determines the speed.

Figures 23 and 24 show different elliptical motions of the contact point.

Figure 23 shows elliptical motions whereby the speed of the slider is changed while maintaining a constant thrust force. Figure 24 shows elliptical motions whereby the thrust force is changed while maintaining a constant speed. An important advantage of this combined phase and amplitude control is that the horizontal speed can be made arbitrary small, while maintaining sufficient thrust force to drive the slider.

This means that there is no 'dead zone⁵ for low speeds, this is an important advantage of this drive over other resonant drives where no independent control of the vertical and the horizontal amplitude is possible.

Other features of the combined motor

Planar version

This motor that combines three operation modes, can be extended to a module suitable for linear movement in two dimensions. Figure 7 shows a preferred embodiment. Two major adjustments have been made compared to the one-dimensional version. Firstly, only the top half of the module is used in order to create enough space for the central fixation to the actuators. Secondly, four pairs of leaf springs have been introduced near the contact point in order to avoid perpendicular motion to both half-ellipses. Active height control

In all three operation modes, the height of the contact point is adjustable: by extending or retracting both actuators, the contact point moves down or up, as seen in figure 8. In the resonant vibration mode, only a part of the voltage range is used to excite the structure. Thus, to each actuator, an arbitrary DC-voltage can be superposed on this excitation signal. This will shift the height of the contact point without a change in the necessary current. This height control can also be applied in the direct positioning mode and in the stepping mode as far as the stroke of the actuators allows it.

This feature is useful when more motors are used to build up a positioning system that integrates bearing and drive function, as described in U.S. Patent 5,898,243. It can be used to compensate for slow disturbances, such as forces or temperature, acting in normal direction on the system. This way, the stiffness of the positioning system can be actively increased.

Bearing function

Because of the stiff fixation to the world and the stiff structure of the stator, the motor can also be used without separate bearings. For example, three planar modules can support a slider. Figure 9 shows the top view of the configuration of the modules and the slider in this case. Note that in this case, it is necessary for the stepping mode to have another three modules (B) that support the slider while the first three modules (A) are retracting into their initial position. Figures 10, 11 and 12 respectively show different possible configurations of the motors in order to construct different positioning systems: an XY- positioning system, a spindle, as described in U.S. Patent 5,898,243 or a Stewart platform.

Tuning blocks

Because of the difficulty to tune the resonance frequency of the two modes of interest so that they lie in the same frequency region, tuning masses are designed to shift the resonant frequency after manufacturing. Figure 7 shows an example of the implementation of these tuning blocks.

Conclusions

Recent technologies require precise positioning systems that are fast, i.e. more than 100 mm/s. An embodiment of present invention is a new kind of motor, which preferably is an electrically-driven motor and more preferably a piezoelectric motor is designed and which achieves this requirements. The motor comprises a stator, preferably an ellipse- shaped stator (e.g. as in fig. 2) or a half-ellipse stator (e.g. as in fig. 8), a single closed structured stator (e.g. as in fig 14), a single half-closed structured stator (e.g. as in fig 15), a doubled closed structured stator (e.g. as in fig. 16), a double half-closed structured stator (e.g. as in fig. 17) or a triple closed structured stator (e.g. as in fig. 18) that can operate in three operation modes. The fact that one motor combines these three operation modes results in a very compact, accurate and fast motor.

As a conclusion, it can be stated present invention has one major advantage compared to prior art, for instance as described in U.S. Pat. No. 6,147,436: the stepping and the direct operation mode can be achieved with only one stator and two actuators, preferably piezoelectric actuators. In U.S. Pat. No. 6,147,436 two ring shaped stators and four piezoelectric actuators are needed for a stepping actuation. Consequently, the motor of present invention can be made more compact and cost-effective than the one proposed in U.S. Pat. No. 6,147,436. Furthermore, the motor of present invention can incorporate the bearing function and moreover the height of the contact point can be independently controlled. This motor is also extendable to a 2D-version (as in fig. 8).

Claims

A MOTOR COMBINING A RESONANT VIBRATION MODE, A STEPPING MODE AND A DIRECT POSITIONING MODECLAIMS

1. A motor, comprising a fixed frame (9, 22, 26 or 31), at least one stator (8, 14, 15, 16, 17, 18, 21, 25 or 32), and at least two actuators (6, 7, 19, 20, 23, 28) in between the stator and the fixed frame and at least one contacting zone for making contact with a mobile unit arranged to move said mobile unit in a sequence of a resonant vibration mode, a stepping mode or a direct positioning mode.

2. The motor of claim 1, wherein said actuators are piezoelectric actuators.

3. The motor of claim 1 or 2, wherein the stator is a closed structure (14), a half-closed structure (15), a double closed structure (16), a double half-closed structure (17) or a triple closed structure (18).

4. The motor of claim 3, wherein the stator has an ellipse-shaped closed structure.

5. The motor of claim 3, wherein the stator has half-ellipse shaped half-closed structure.

6. The motor of any of the claims 3 to 5, wherein the stator comprises tuning masses.

7. The motor of any of the claims 3 to 6, wherein the stator is a monolithic structure.

8. The motor of claim 7, where said monolithic structure is manufactured by electron discharge machining or selective laser sintering.

9. An apparatus comprising the motor of any of the claims 1 to 8 and a mobile unit, wherein the mobile unit is driven by the motor.

10. The apparatus of claim 9, wherein the motor (12) is a bearing system of the mobile unit (13).

11. The apparatus of claim 9, wherein the motor is contacting the mobile unit supported on separate bearings.

12. The apparatus of any of the claims 9 to 11, wherein the mobile unit comprises at least one slider, wheel or a ball which makes contact with the stator of the motor.

13. The motor of claim 1, arranged to control the thrust force of the mobile unit and the speed of the mobile unit independently.